The invention relates to a process and device for shaping surfaces, in particular lenses, by laser ablation of the surfaces, particularly but not exclusively, surfaces of biological material. The invention preferably is applied in photorefractive keratectomy (PRK) and in ophthalmological precision shaping of contact lenses.
The known state of art will be described hereinafter with reference to processes and devices for shaping surfaces relating to the correction of deficiency of sight and and which are most relevant to the present invention. The ablation processes in the treatment of deficiencies of sight of the human eye which have been carried out since the middle 1980's are all based on formulas for calculating the required flattening and steepening, respectively, of the cornea as first described by Munnerlin. When correcting myopia and according to the above calculation, more corneal tissue is ablated in the center of the cornea than in the peripheral ranges of the cornea. When correcting hyperopia, more tissue is ablated in the periphery of the cornea than in the center of the cornea. The enhanced or reduced effect caused in this manner with respect to the power refraction of the cornea surface corresponds to that of a contact lens.
In previous processes closely related to the present invention, the ablation of a respective amount of cornea is carried out in individual successive ranges of constantly varying areas. In the theoretically simplest case, namely the correction of myopia, the areas are described by successive circles of continuously decreasing or increasing diameters. To obtain an optimal result of treatment, it is necessary to completely ablate all layers in a series. In the presently known procedures, such series are worked off in a continuously increasing or decreasing sequence from the beginning to the end. When, while performing such procedures, there is an unintentional break-off of the treatment, anomalous cornea surfaces will result which generally lead to a considerable deterioration of eyesight. Regrettably, such defects of sight can be removed only partially or not at all by conventional means such as glasses or contact lenses. The only way to correct the situation will be to continue treatment at exactly that point where it was interrupted. When the exact point cannot be found or the patient is not ready to undergo a further treatment, a permanent deterioration of the eyesight will result.
A detailed description of the state of art is disclosed in European Patent Application No. 90308709.6 (corresponding to German Patent Publication No. DE 690 24 558 T2).
U.S. Pat. No. 5,520,679 describes an ophthalmic surgery method using a spot scanning laser in which the cornea ablation is achieved by setting single laser spots. This method, which operates like the ablation processes mentioned hereinbefore, tries to achieve an ablation as uniform as possible by setting laser spots in close timely succession with a defined overlap rate. This, however, involves an increased thermal stress to individual ranges of area, in particular, when an erbium laser is employed.